A plant for power transmission by means of high voltage direct current comprises two convertor stations interconnected by a d.c. line. In a single-pole d.c. transmission, the stations are interconnected by means of one single d.c. conductor, the return current being conducted through ground. One d.c. pole in each station is then connected to ground by means of an efficient ground terminal. Normally, this is arranged at a distance from the convertor station and connected to the station via a so-called electrode line. It may often be desirable or necessary to locate the ground terminal at a long distance from the station, up to several hundred kilometers from the station.
In a so-called bipolar d.c. transmission, the stations are interconnected by means of two d.c. conductors and in normal operation, therefore, the direct current need not be returned through ground. For several reasons, among other things to make possible single-pole operation of the plant in case of a convertor failure, also convertor stations in bipolar transmissions are provided with a ground terminal which is connected to the station by means of an electrode line.
An electrode line is insulated relative to ground and normally consists of a pole line suspended from insulators. Even if the voltage of the electrode line to ground is normally low in relation to other voltages in the plant, a ground fault on the electrode line causes a risk of personal injuries and of damage to other plant components, for example corrosion damage. It is therefore important that ground faults, also high-ohmic ground faults, can be rapidly and effectively detected.
Proposals have been made to use a differential protective device for detection of ground faults in an electrode line. In such a protective device, the current at both ends of the electrode line is measured, and a difference appearing between the two measured currents is an indication of a ground fault. However, such a protective device has several drawbacks. It requires a communication link between the two ends of the electrode line and therefore, especially in connection with long electrode lines, becomes expensive and not fully reliable. Further, it has been found to be difficult or impossible to design such a protective device to become capable of detecting high-ohmic ground faults. Nor does a protective device of this kind react on a ground fault occurring in those cases in which the electrode line does not carry any current, which is normally the case in undistrubed operation of a bipolar transmission. Also in this case, i.e. is when no direct current flows through the electrode line, harmonic currents may give rise to dangerous voltages on the line.
It has furthermore been proposed to detect ground faults on an electrode line by injecting, at the convertor station, an alternating current or alternating voltage signal of a predetermined frequency on the line. Suppression filters are then arranged at the two ends of the line, these filters being tuned to the injection frequency. An impedance measuring device is arranged to measure the impedance of the electrode line at the feeding point in relation to ground at the injection frequency. A change of the impedance thus measured is an indication of a ground fault. This method functions well in the case of short electrode lines but exhibits drawbacks in long electrode lines. Since it is necessary to select the measuring frequency so low that standing waves on the electrode lines are avoided, the length of the line must be less than one-fourth of a wavelength at the current frequency. In the case of long electrode lines, for this reason, such a low frequency must be selected that there will be a risk that the measurement is disturbed by the mains frequency or by the lowest harmonics of the mains frequency. Further, at these low frequencies the suppression filters, which are arranged at both ends of the electrode line and which must be dimensioned for maximum electrode line current, will be large and expensive.